1. Field of the Invention
The present invention relates to electrical power supply or regulation systems, and more particularly, to current limiting devices for such systems. In particular, the present invention relates to a circuit providing current limiting capabilities to limit in-rush current and current surges during operation.
2. Background of the Invention
Power supplies for computers and other digital devices typically accept an AC current source, and convert it into a DC current source for operation of the device. The AC source is rectified to produce high voltage DC power which is regulated to the voltage level necessary for operation of the electronic device. A simplified diagram of such a power supply 90 is shown in FIG. 1. The AC line current is connected to rectifier 100 that produces rectified DC voltage which appears across the DC load 102. Filter capacitor 104 is provided to smooth the regulated DC voltage, ensuring a constant voltage across load 102.
When the power supply 90 is first turned on, the voltage across capacitor 104 will be zero while the voltage at the output of rectifier 100 will be the maximum produced by the AC line source. The difference between the AC line source voltage and voltage across capacitor 104 can be represented as VMAX. The initial large difference in voltage will create high current flows through the circuit as capacitor 104 is charged. These high currents may damage components in the power supply or cause disruption to the AC line source.
The prior art has dealt with this problem by introducing current limiting devices such as a current limiting resistor 106. Such devices prevent the in-rush current from exceeding a predetermined level and prevents damage to the components. The disadvantage of using a current limiting resistor is that the overall circuit efficiency is reduced due to the IR drop of the resistor. Thus, while the resistor is useful to control in-rush current surges, it decreases the efficiency of the circuit in steady state operation. Also, the circuit has poor voltage regulation as a function of load current.
One approach to solving the inefficiency problem is to use a negative temperature coefficient thermistor as the current limiting device. The thermistor resistance decreases as the device temperature increases. Thus, after the power supply has reached the steady state and the thermistor heat has increased, the resistance will be very low, providing an efficient circuit. The disadvantage of this device is that it will not reduce current surges caused by voltage surges at the AC source 101. Thus, the thermistor provides little circuit protection against voltage surges.
A second approach to limiting the efficiency impact of current limiter 106 is to introduce a switch 108 that provides a lower resistance circuit in parallel with the current limiting resistor. The switch, such as an SCR, is activated once the circuit reaches steady state so that the current limiting device is switched out of the circuit at steady state. This provides the advantages of an efficient circuit but still provides no protection against current surges occurring after startup.
Thus, there is a need for a current limiting circuit able to control both in-rush current and current surges caused by AC line voltage surges, and which operates efficiently as part of the power supply.